I was having a hard time getting my head around the way interrupts work and I looked in the 'Arduino Cookbook' and found a reference to a library called MsTimer2 (http://www.arduino.cc/playground/Main/MsTimer2). This is much simpler. I just need two functions, one to set the tempo (beats per minute) and one to send the sync.

This works with Ableton Live.

Thanks, Dr Speed

#include <MsTimer2.h>

define PPQ 24

// when the sequencer starts, set the timer with the new tempo, something like 75 or 125, a normal bpm.

void setTimer(int beatsPerMinute){

// sync period in milliSeconds period = ((1000L * 60)/beatsPerMinute)/PPQ;